The present invention is directed generally to protecting an individual's tissues and cells against the damaging effects of an agent that elicits the production of a free radical, superoxide anion, or heavy metal cation when that individual is exposed to an agent. Specifically, the invention is directed to obtaining this protection by transient expression of a protective protein through somatic gene transfer in vivo.
Therapeutic concentrations of anti-cancer drugs and clinical radiation therapy are known to damage a patient's normal tissues and cells. A need clearly exists for means to protect a patient's normal tissues during chemotherapy and/or radiation therapy. Previous methods of affording such protection include administration of sulfhydryl compounds such as thiols or other radical scavenger compounds.
The major way in which radiation damages biomolecules and cells is through its interaction with water to produce toxic free radicals (H.sup..cndot., OH.sup..cndot., e.sub.aq.sup.-) and H.sub.2 O.sub.2 or, through interaction with oxygen, to produce the superoxide radicals (.sup..cndot. O.sub.2.sup.-). In the late 1940's it was discovered that sulfhydryl compounds, such as cysteine and cysteamine, provide radiation protection in animals. Patt et al., Science 110: 213 (1949). Thiol groups scavenge radiation-produced free radicals by donating a hydrogen atom to damaged molecules. Despite extensive efforts to develop more effective protective agents, no thiol-based radioprotector has been found to be significantly better than cysteamine. Mitchell et al., Arch. Biochem. and Biophys. 289: 62 (1991). However, the use of thiol drugs to protect against radiation damage is limited by the toxicity of such compounds.
Antineoplastic agents, particularly the class of chemotherapeutic drugs known as alkylating agents, also produce free radicals that are cytotoxic due to their ability to form covalent bonds with nucleic acids. Most alkylating agents form positively charged carbonium ions that yield the charged alkylating intermediate R--CH.sub.2 --CH.sub.2.sup.+ that attacks electron-rich sites on nucleic acids, proteins, small molecules and amino acids.
Several endogenous intracellular scavengers of free radicals, superoxide radicals and heavy metal cations have been identified. Induction or elevated activities of each of metallothionein (MT), gamma-glutamyl transpeptidase (.gamma.-GTP) and superoxide dismutase (SOD) are known to provide resistance to ionizing radiation damage in vitro. Since these proteins function intracellularly to scavenge free radicals, superoxide anions or heavy metal cations, these proteins need to be provided or induced intracellularly. Administration of these proteins directly cannot provide continual levels of the intracellular quantities required to furnish protection against ionizing radiation or an anticancer agent. Furthermore, if metallothionein, superoxide dismutase or gamma glutamyl transpeptidase proteins are administered to cells extracellularly, they may be rapidly degraded by proteases and fail to function intracellularly. No method for providing functional intracellular therapeutic levels of metallothionein, superoxide dismutase or gamma glutamyl transpeptidase to normal tissues in vivo is known.